The overall objective of the proposed research is to elucidate further the skeletal muscle growth of chickens at different ages and to assess endocrine influences on the muscle growth during these different stages of growth. The following parameters of muscle growth are studied in individual skeletal muscles: fresh weight and the levels of crude fibrillar protein, actin and DNA. Our studies have focused on the thyroidal influence on muscle growth from hatching to nine weeks of age and during embryogenesis from 10 to 20 days of incubation. In addition to further studies of the role of thyroid hormone in muscle growth, we propose to examine the effects of mammalian and avian growth hormone preparations on muscle growth.